I've been participating (once again*) in a Facebook discussion among people who claim that the MBTI is not a "valid" assessment because some people get a different four-letter code the second time they take the inventory. Many people can't bear the idea that the reason for a different result is as simple as the fact that they answered the questions differently. They don't want to investigate why that happened, so they blame "the test".

For people who don't understand, I'll be specific. The MBTI Inventory asks questions about your preferences. All it measures is your responses to the questions. If your answers change, your result changes.

The MBTI will give you the same Type code every time if you answer the questions the same way. Millions of people do answer the questions the same way. "It should be understood that the MBTI® instrument meets and exceeds the standards for psychological instruments in terms of its reliability. (ref)".

Perhaps because it involves humans, however, many people attribute "different results" for the MBTI to a fault in the assessment ("it asks bad questions!"). The fault obviously cannot lie with the person answering, i.e. "I can't possibly be inconsistent in my understanding of my internal preferences!"

If we make an analogy between the MBTI and a thermometer, your answers are analogous to a cup of tea. The measured temperature of the tea can vary, depending on when it's poured, the temperature of the room, the type of cup, how long ago the tea was poured, the temperature of the water used, and many other factors.

If you think that you can use the same thermometer on many cups of tea and always get the same result, you're not thinking clearly. This only works if you very carefully regulate the properties of the cups of tea.

If your MBTI results change, that means that the "you" answering the questions today is not responding the same way as the "you" answering the identical questions six months (or whenever) ago.
If your result changes, you should not claim that the MBTI is unreliable. Instead, the much more interesting question you should ask is, "why did my answers change?

I have been having a "discussion" in The Science Enthusiasts group in Facebook. Someone posted another "debunking the MBTI" article (from 2014). Surprisingly, this one doesn't cite "Wharton Scholar, Adam Grant".

The article includes this statement: "Several analyses have shown the test is totally ineffective at predicting people's success in various jobs, and that about half of the people who take it twice get different results each time."

Well, yes. Both of those things are, at least partially, true. Neither, however, is an argument that "debunks" the MBTI.

My understanding is that Jung did not mean for his theories to be used as "labels." He meant them to help people grow and develop beyond a way of being. I have often heard people say things like, "I'm an INFP" or something similar. It makes me think people take these letters as a personality label and I am uncomfortable with that rigid interpretation.

Now, first of all, we should note that the phrasing used in the question is "I'm an INFP" or something similar. There's a big difference between a label given and a label accepted.

Present type preferences as tendencies, preferences, or inclinations, rather than absolutes. Though people have preferences and predispositions, all people have every capacity and function described in the theory of type.

(They leave out the fact that all people do not have equal access to or comfort with every capacity and function.... But I digress.)

That said, it can become cumbersome to say "People with a preference for INFP tend to ..." every time. When everyone knows the language, it can often be a lot simpler to use shorthand wording, e.g. "INFP tend to ...". Even 'INFP' is itself a shorthand for "dominant introverted feeling and auxiliary extraverted intuition". This is why language shorthand (i.e. jargon) was invented.

The MBTI (Myers-Briggs Type Indicator) is a self-assessment and self-discovery instrument for Psychological Type. It was developed in the late 1940s and has been in use since the 1960s. To date, millions of people have taken the MBTI. The questions have been reconsidered and rewritten multiple times, based on how well they sort people into "Type". The assessment currently in use is "Form M".

Notwithstanding its popularity (or perhaps because of it), there are detractors. One of the loudest of these, Adam Grant, bases most of his dislike for the MBTI on the fact that he has gotten inconsistent results. In other words, he interprets his ability to "trick" a self-assessment tool, by supplying different answers each time, as a flaw in the tool. (I lost any respect for Mr. Grant after reading his essay on the topic.)

Another common method of refuting the MBTI is a version of what techies call "Not Invented Here" syndrome. Simply put, the creators of the MBTI were not psychologists.